1. Field of the Invention
The present invention relates to a projection exposure apparatus used to transfer a mask pattern onto a photosensitive substrate in a photolithography process for producing, for example, semiconductor integrated circuits, liquid crystal display devices, imaging devices (e.g. CCDs), or thin-film magnetic heads. More particularly, the present invention relates to a projection exposure apparatus which is suitable for carrying out exposure for different layers on a photosensitive substrate by the mix-and-match method.
2. Related Background Art
In the production of semiconductor devices, for example, projection exposure apparatuses (e.g. steppers) have heretofore been used to transfer a pattern formed on a reticle, which is used as a mask, onto a photoresist-coated wafer through a projection optical system. In general, a semiconductor device is formed by stacking a plurality of layers of circuit patterns on a wafer with a predetermined positional relationship. In many of recent semiconductor manufacturing factories, exposure is carried out by the mix-and-match method mixedly using different projection exposure apparatuses for different layers on a wafer in order to increase the throughput (i.e. the number of wafers processed per unit time). Projection exposure apparatuses used in this process are stringently required to provide high overlay accuracy. Accordingly, each projection exposure apparatus is accurately corrected for image distortion such as distortion associated with a projection optical system. However, it is difficult to completely correct image distortion; some image distortion remains uncorrected. In this case, there may be intolerable variations of image distortion between projection exposure apparatuses even if the image distortion for each apparatus is within tolerance. Particularly, when projection exposure apparatuses differ from each other in the magnification of their projection optical systems and in the size of the exposure field, or when a system mixedly uses projection exposure apparatuses which have different dates of manufacture and hence are different in the tolerance standard for image distortion, there are image distortion variations as described above, and the overlay accuracy, i.e. matching accuracy, reduces.
To solve the above-described problems, Japanese Patent Application Unexamined Publication (KOKAI) Nos. 62-7129 and 62-24624 disclose an exposure method wherein the projection magnification of each apparatus is adjusted such that the overlay accuracy of a distorted image is optimized in each apparatus, and the exposure position is also corrected so as to be optimized. Japanese Patent Application Unexamined Publication (KOKAI) No. 4-127514 discloses a method wherein a reticle or an optical member, which forms a projection optical system, is three-dimensionally moved or tilted to correct anisotropic image distortion, thereby further improving the overlay accuracy. Japanese Patent Application Unexamined Publication (KOKAI) No. 5-166699 discloses a method wherein information concerning distortion in each projection optical system is controlled by a central processing unit so as to obtain the highest distortion matching accuracy between projection exposure apparatuses.
Image distortion components that can be corrected by the above-described conventional techniques are only predetermined symmetric components such as translation components correctable by correction of the exposure position, point symmetry components correctable by correction of the projection magnification, or line symmetry components correctable by correction of the tilt angle of an optical member in the projection optical system. Conventionally, attempts have been made to reduce the overlay error by making full use of techniques whereby such symmetric components alone are corrected. Therefore, correction has heretofore been made, for example, by calculating correction conditions which minimize the largest error in the amounts of displacement of a projected image relative to ideal lattice points, that is, the largest error of so-called distortion errors.
However, even if such a correction is made, it is impossible to avoid some image distortion in a projection optical system which consists of asymmetric random components due to refractive index nonuniformity based on the heterogeneity of a glass material which is difficult to remove or due to a local deviation of an optical member from a spherical surface caused during glass polishing process. In the past, it was rare for such random components to give rise to a problem in terms of the overlay accuracy. In recent years, however, as the pattern line width has become increasingly fine, the required overlay accuracy has become strict, and such random image distortion components have become a hindrance to the attainment of the required overlay accuracy.
When projection exposure apparatuses different from each other in the field angle (i.e. the size of the exposure field) are mixedly used, image distortion which has symmetry in each particular projection exposure apparatus may develop asymmetric image distortion in another projection exposure apparatus which differs from the first-mentioned apparatus in the field angle. In this case also, it is difficult with conventional techniques to obtain satisfactorily high matching accuracy, and similar problems arise.